Diamond-copper composites were prepared by high temperature and high pressure method (HPHT). The effects of diamond volume fraction, sintering pressure, holding time, sintering temperature, surface metallization of diamond on thermal conductivity and thermal expansion coefficient of diamond-copper composites were studied. The experimental results show that when diamond volume fraction is 70%, sintering pressure is 2 GPa, sintering time is 300 s, and sintering temperature is 1200℃, the thermal conductivity of diamond-copper composite material reaches 426 W/(m·K).